Golf ball

ABSTRACT

A golf ball having a cured polyurethane cover which includes an isocyanate group terminated polyurethane prepolymer comprising a specific alicyclic diisocyanate as a polyisocyanate component, and a specific 4,4′-diaminodiphenylmethane derivative as a curing agent.

BACKGROUND OF THE INVENTION

This application is based on Japanese Patent application No.2002-170964filed on Jun. 12, 2002, the contents of which are hereby incorporated byreference.

FIELD OF THE INVENTION

The present invention relates to a golf ball having a polyurethanecover, more particularly, to a golf ball which comprises a solid coreand the polyurethane cover covering the solid core, and which exhibitsan excellent durability and coloring-resistance against weather withoutlowering the productivity of the polyurethane cover.

DESCRIPTION OF THE RELATED ART

As a conventional golf ball, a wound-core golf ball having a “Balata”cover, or a multi-piece golf ball having an ionomer cover is well-known.The wound-core golf ball having a Balata cover is widely used byhigh-level golfers and professional golfers, because Balata cover issuperior in feel and control. Herein, “feel” is the overall sensation tothe golfer when the golf ball is hit, and “control” is to impart themotion such as backspin to the golf ball.

However, the golf ball having a Balata cover is expensive due to acomplicated manufacturing process and is inferior in durability and cutresistance. On the other hand, the golf ball having an ionomer resincover is also widely used, because of good durability andflight-performance. However, it has been pointed out that the ionomerresin cover is inferior in feel and control to that of the Balata cover.

In view of these circumstances, a golf ball having a polyurethane coverhas been proposed. It is expected that a golf ball having a polyurethanecover exhibits the feel and control which the Balata cover imparts aswell as the durability and flight performance which the ionomer coverachieves.

The two-component type polyurethane cover is formed by curing apolyurethane cover composition comprising an isocyanate group terminatedpolyurethane prepolymer and a polyamine as a curing agent. In thistwo-component system, it is difficult to adjust the reactivity betweenthe isocyanate group terminated polyurethane prepolymer and thepolyamine, thus resulting in problems of productivity and performance ofthe golf ball. For example, if the reactivity between the isocyanategroup and the amino group is too high, the polyurethane covercomposition will have the increased viscosity, which sometimes leads togelation. Thus, it is impossible to form such a polyurethane cover. Onthe other hand, if the reactivity is too low, it is necessary to spendlonger time on curing the polyurethane cover composition, resulting inthe lower productivity. For example, in Japanese Patent No.2,662,909,the reactivity between the isocyanate group terminated polyurethaneprepolymer and the polyamine is adjusted by using a polyamine having alow reactivity. The above polyamine having a low reactivity isconventionally used for curing the aromatic polyisocyanate polyurethaneprepolymer, such as 4,4′-diphenylmethanediisocyanate, which has a highreactivity. However, the polyurethane cover containing the aromaticpolyisocyanate has low weather-resistance, thus yellowing occurs.

The approach to overcome the yellowing of the polyurethane cover is theuse of the aliphatic or alicyclic polyisocyanate. However, since thecohesive energy of the aliphatic or alicyclic polyisocyanate is lowerthan that of the aromatic polyisocyanate, the mechanical properties ofthe obtained polyurethane cover tends to be lowered. Thus, if thearomatic polyisocyanate is just replaced with the aliphaticpolyisocyanate or alicyclic polyisocyanate, a polyurethane cover whichexhibits a satisfactory performance cannot be obtained. Further, asdescribed above, if the polyamine having a low reactivity, which isconventionally used for the aromatic polyisocyanate, is substituted forthe aliphatic or alicyclic polyisocyanate, the reactivity of thepolyamine is too low for the aliphatic or alicyclic polyisocyanate,resulting in a lowered productivity.

SUMMARY OF THE INVENTION

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description.

The present invention has been achieved to overcome the above problems.An object of the present invention is to provide a golf ball having thepolyurethane cover, which exhibits excellent durability andcoloring-resistance against weather without lowering the productivity ofthe polyurethane cover.

The present invention provides a golf ball having a polyurethane cover,wherein the polyurethane cover is formed by curing a polyurethane covercomposition which comprises

an isocyanate group terminated polyurethane prepolymer comprising atleast one diisocyanate selected from the group consisting of4,4′-dicyclohexylmethanediisocyanate, isophoronediisocyanate, andcyclohexanediisocyanate as a polyisocyanate component, and

a 4,4′-diaminodiphenylmethane derivative represented by the followingformula (1).

In the formula (1), R¹ to R⁸ represent any one of an alkyl group having1 to 3 carbons carbon atoms or a hydrogen atom, and each of R¹ to R⁸ maybe same as or different from other.)

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

The present invention provides a golf ball having a polyurethane cover,more preferably, a golf ball having the polyurethane cover whichcomprises a solid core and the polyurethane cover covering the solidcore.

The polyurethane cover used in the present invention is formed by curinga polyurethane cover composition which comprises

an isocyanate group terminated polyurethane prepolymer (hereinafter maybe just referred to as “isocyanate prepolymer”) comprising at least onediisocyanate selected from the group consisting of4,4′-dicyclohexylmethanediisocyanate, isophoronediisocyanate, andcyclohexanediisocyanate as a polyisocyanate component, and

a 4,4′-diaminodiphenylmethane derivative represented by the followingformula (1).

In the formula (1), R¹ to R⁸ represent any one of an alkyl group having1 to 3 carbon atoms, or a hydrogen atom, and each of R¹ to R⁸ may be thesame as or different from each other.

First of all, the isocyanate group terminated polyurethane prepolymerwill be described. The isocyanate group terminated polyurethaneprepolymer used for the present invention is not limited, as long as ithas at least two isocyanate groups in a polyurethane prepolymermolecular chain and contains at least one diisocyanate selected from thegroup consisting of 4,4′-dicyclohexylmethanediisocyanate,isophoronediisocyanate, and cyclohexanediisocyanate as an polyisocyanatecomponent. The location of each isocyanate group in a polyurethaneprepolymer molecular chain is not particularly limited. The isocyanategroup may be located at the terminal of the backbone chain or side chainof the polyurethane prepolymer molecular chain. In the presentinvention, at least one diisocyanate selected from the group consistingof 4,4′-dicyclohexylmethanediisocyanate, isophoronediisocyanate, andcyclohexanediisocyanate is used as polyisocyanate component, becausethese diisocyanates do not cause yellowing, which the aromaticpolyisocyanate dose. Among the above diisocyanates, it is preferable touse 4,4′-dicyclohexylmethanediisocyanate(hydrogenated MDI). Because4,4′-dicyclohexylmethanediisocyanate used as the polyisocyanatecomponent imparts abrasion-resistance as well as yellowing-resistance.

The reason why 4,4′-dicyclohexylmethanediisocyanate improves theabrasion-resistance is not apparent, but it is considered that thecohesive energy of the polymer chains of the resultant polyurethanecover is enhanced, since the molecules of4,4′-dicyclochexylmethane-diisocyanate and the4,4′-diaminodiphenylmethane derivative as a curing agent align easily.

In a more preferable embodiment, the polyisocyanate component of theisocyanate group terminated polyurethane prepolymer essentially consistsof at least one diisocyanate selected from the group consisting of4,4′-dicyclohexylmethanediisocyanate, isophoronediisocyanate, andcyclohexanediisocyanate.

The isocyanate group terminated polyurethane prepolymer, withoutlimitation, can further comprise a polyol component. For example, theisocyanate prepolymer can be obtained by reacting at least onediisocyanate selected from the group consisting of4,4′-dicyclohexylmethanediisocyanate, isophoronediisocyanate, andcyclohexanediisocyanate with the polyol so that isocyanate group of thediisocyanate is excess to the hydroxyl group of the polyol in a molarratio.

The polyol contained in the isocyanate prepolymer may be any polyol, aslong as the polyol has a plurality of hydroxyl groups. The polyol has nolimitation on its molecular weight, and may include alow-molecular-weight polyol or a high-molecular-weight polyol.

The low-molecular-weight polyol includes, for example, a diol such asethylene glycol, diethylene glycol, triethylene glycol, 1,3-butanediol,1,4-butanediol, neopentyl glycol, and 1,6-hexanediol; and a triol suchas glycerin, trimethylolpropane, and hexanetriol. Thehigh-molecular-weight polyol includes, for example, a polyether polyolsuch as polyoxyethyleneglycol (PEG), polyoxypropyleneglycol (PPG) andpolyoxytetramethyleneglycol (PTMG); a polyester polyol such aspolyethyleneadipate, polybutyleneadipate, polyhexamethyleneadipate(PMHA); a lactone polyester polyol such as ε-caprolactone; apolycarbonate diol such as polyhexamethylenecarbonate; and a polymerpolyol such as an acrylic polyol. Among them, typically preferred is apolyether polyol, more preferably, a polyoxytetramethyleneglycol in viewof imparting the superior impact resilience and water resistance.

Secondly, the 4,4′-diaminodiphenylmethane derivative represented by thefollowing general formula (1) will be described below.

In the formula, R¹ to R⁸ represent any one an of alkyl group which has 1to 3 carbon atoms, or a hydrogen atom, respectively, and each of R¹ toR⁸ may be the same as or different from each other.In the present invention, the above 4,4′-diaminodiphenylmethanederivative is used as the curing agent for the isocyanate prepolymercontained in the polyurethane cover composition, because the reactivitythereof is suitable for the alicyclic polyisocyanate such as4,4′-dicyclohexylmethanediisocyanate, isophoronediisocyanate, andcyclohexanediisocyanate.In a preferable embodiment, the curing agent for the isocyanateprepolymer substantially consists of the above4,4′-diaminodiphenylmethane derivative represented by the formula (1).The use of the 4,4′-diaminodiphenylmethane derivative makes the pot lifeof the polyurethane cover composition from 30 to 900 seconds, therebyimproving the workability for curing the polyurethane cover composition.Further, since the use of 4,4′-diaminodiphenylmethane derivativeprovides a high cohesive energy, even if the aliphatic or alicyclicpolyisocyanate is used in place of the aromatic polyisocyanate, it ispossible to obtain a polyurethane cover having excellent durability andmechanical properties. The reactivity of 4,4′-diaminodiphenylmethanederivative to the isocyanate group is adjusted by the structure of thesubstituent groups of R¹ to R⁸ bonded to the aromatic ring. In thepresent invention, the substituent group of R¹ to R⁸ includes a hydrogenatom or alkyl group having 1 to 3 carbons. Each of R¹ to R⁸ may be thesame as or different from each other. The reason why the alkyl grouphaving 1 to 3 carbon atoms is used is that the reactivity of the aminogroup becomes too low, due to the large steric hindrance of thesubstituent groups, when having carbon atoms of not less than 4. Also,it is preferable to use 4,4′-diaminodiphenylmethane derivative which ismore reactive to the isocyanate group than3,3′-dichloro-4,4′-diaminodiphenylmethane (MOCA). Thehalogen-substituted diaminodiphenylmethane such as MOCA, of which thereactivity to the isocyanate group is reduced by halogen-substitution,is suitable as a curing agent for the aromatic polyisocyanate. Thus, ifthe halogen-substituted diaminodiphenylmethane is used as a curing agentfor the aliphatic or alicyclic polyisocyanate, the reaction therebetweenis too slow. The relative reactivity of 4,4′-diaminodiphenylmethanederivative is, for example, described as follows (“Polyurethane ResinHandbook (Nikkan Kogyosya, Keiji, Iwata).

TABLE 1 Relative reactivities of 4,4′-diaminodipheylmethane (DADPM)derivative Relative Structures of alkyl groups reactivity3,3′-dichloro-4,4′-DADPM(MOCA) 1 3,3′,5,5′-tetramethyl-4,4′-DADPM 173,3′5,5′-tetraethyl-4,4′-DADPM 9 3,3′,5,5′-tetraisopropyl-4,4′-DADPM 183,3′-dimethyl-5,5′-diisopropyl-4,4′-DADPM 18

Examples of the alkyl group having 1 to 3 carbons are methyl group,ethyl group, n-propyl group, and isopropyl group. Examples of4,4′-diphenylmethane derivative are 4,4′-diaminodiphenylmethane, or analkyl-substituted 4,4′-diphenylmethane such as2,2′-dimethyl-4,4′-diaminodiphenylmethane3,3′-dimethyl-4,4′-diaminodiphenylmethane3,3′,5,5′-tetramethyl-4,4′-diaminodiphenylmethane,2,2′-diethyl-4,4′-diaminodiphenylmethane3,3′-diethyl-4,4′-diaminodiphenylmethane3,3′,5,5′-tetraethyl-4,4′-diaminodiphenylmethane,2,2′-dipropyl-4,4′-diaminodiphenylmethane3,3′-dipropyl-4,4′-diaminodiphenylmethane3,3′,5,5′-tetrapropyl-4,4′-diaminodiphenylmethane, or3,3′-diethyl-5,5′-dimethyl-4,4′-diaminodiphenylmethane. Among them,preferred is 3,3′,5,5′-tetralkyl-4,4′-diaminodiphenylmethane, morepreferred is 3,3′-diethyl-5,5′-dimethyl-4,4′-diaminodiphenylmethane.

The polyurethane cover composition used in the present inventionincludes the above isocyanate group terminated polyurethane prepolymerand the above 4,4′-diaminodihphenylmethane derivative. The polyurethanecover composition is cured to form the polyurethane cover. Theisocyanate group terminated polyurethane prepolymer and the4,4′-diaminodiphenylmethane derivative are contained in the polyurethanecover composition in a molar ratio of the amino group to the isocyanategroup (NH₂/NCO) preferably ranging from 0.70, more preferably from 0.80,even more preferably from 0.85, and preferably to 1.20, more preferablyto 1.05, even more preferably to 1.00. If the ratio is less than 0.70,the amount of the isocyanate prepolymer to 4,4′-diaminodiphenylmethanederivative become excess, thus the alophanate or biruet bond tends togenerate excessively. The excess alophanate or biruet bond causes thelack of softness of the resultant polyurethane cover. On the other hand,if the ratio is more than 1.20, since the isocyanate group is lacking,it becomes difficult to generate the alophanate or biruet bond. As aresult, the amount of the three-dimensional crosslinking points becomestoo low, resulting in the poor strength of the obtained polyurethanecover.

In the present invention, the polyurethane cover composition preferablyhas a pot life of not less than 30 seconds, more preferably not, lessthan 40 seconds, and not more than 900 seconds, more preferably not morethan 600 seconds. Herein, the term “pot life” means a time periodbetween the time the isocyanate prepolymer heated at 80° C. and4,4′-diaminodiphenylmethane derivative are mixed and the time when theviscosity of the polyurethane cover composition reaches 20,000 mPa·s(cPs). Furthermore, when the isocyanate prepolymer and4,4′-diaminodiphenylmethane derivative are mixed,4,4′-diaminodiphenylmethane derivative is heated to the predeterminedtemperature (Curing agent adding temperature) prior to the mixing, andthen mixed with the isocyanate prepolymer. For example, if4,4′-diaminodiphenylmethane derivative is liquid at the roomtemperature, it can be mixed at the room temperature. In the case that4,4′-diaminodiphenylmethane derivative is solid at the room temperature,it may be heated at a temperature which is higher than the melting pointthereof by 10 to 20° C., and then the 4,4′-diaminodipheylmethanederivative in a liquid state may be mixed. The pot life is an indicatorof the reactivity of the curing reaction between the isocyanateprepolymer and 4,4′-diaminodiphenylmethane derivative.

If the pot life is shorter than 30 seconds, a sufficient operating timecannot be ensured, because the curing reaction is too fast. If the potlife is longer than 900 seconds, curing time becomes longer, resultingin the lowered productivity. The polyurethane cover compositionpreferably has the viscosity of not more than 20,000 mPa·s (cPs). If theviscosity is more than 20,000 mPa·s (cPs), the fluidity of thepolyurethane cover composition is lowered, resulting in the loweredworkability. The viscosity of the polyurethane cover composition can bemeasured by using a Brookfield type viscometer.

The polyurethane cover composition for use in the present invention mayinclude the conventional catalyst which is well-known for the reactionof the polyurethane, in addition to the isocyanate prepolymer and4,4′-diaminodiphenylmethane derivative. Examples of the catalyst are amonoamine such as triethylamine and N,N-dimethylcyclohexyl amine; apolyamine such as N,N,N′,N′-tetramethylethylene diamine andN,N,N′,N″,N″-pentamethyldiethylene triamine; a cyclic diamine such as1,8-diazabicyclo[5,4,0]-7-undecene (DBU) and triethylene diamine; and atin catalyst such as dibutyltin dilaurylate and dibutyltin diacetate.Among these catalysts, it is preferable to use1,8-diazabicyclo[5,4,0]-7-undecene (DBU) and triethylene diamine.

As required, the polyurethane cover composition may further contain afiller such as barium sulfate, a pigment such as titanium dioxide, andother additives such as a dispersant, an antioxidant, an ultravioletabsorber, a light stabilizer, and a fluorescent material or afluorescent brightener, unless they impair any undesirable property tothe intended golf ball.

The golf ball of the present invention can be obtained by covering asolid core with the polyurethane cover composition, and then curing thepolyurethane cover composition. The conditions for curing thepolyurethane cover composition depend on the each type of the isocyanateprepolymer and 4,4′-diaminodiphenylmethane derivative. Generally, thepolyurethane cover composition is preferably cured at the temperaturebetween the room temperature and 140° C., for 1 to 60 minutes. By curingat the above condition, it is possible to reduce the influence offorming the polyurethane cover on the molded-rubber and improve theproductivity and workability.

The polyurethane cover preferably has a Shore D hardness of not lessthan 30, more preferably not less than 35, and a Shore D hardness of notmore than 60, more preferably not more than 55. If the polyurethanecover has the Shore D hardness of larger than 60, the polyurethane coverbecomes too hard, resulting in the larger impact when hit. Further, thecontrollability of the obtained golf ball becomes poor, due to the lowerspin rate. On the other hand, if the polyurethane cover has a Shore Dhardness of less than 30, the polyurethane cover will be too soft,resulting in the low resilience.

The polyurethane cover preferably has a thickness of not less than 0.2mm, more preferably not less than 0.3 mm, and preferably not more than1.5 mm, more preferably not more than 1.3 mm. By making the thickness ofthe polyurethane cover thinner than 1.5 mm, which is thinner than thatof the conventional cover, it is possible to prevent the resilience frombeing lowered by employing the relatively soft polyurethane cover.However, if the thickness of the polyurethane cover is less than 0.2 mm,since the thickness is too thin, it will be difficult to form thepolyurethane cover. Even if the polyurethane cover having the thicknessof less than 0.2 mm can be formed, the tear-strength of the resultantpolyurethane cover is low. Thus, the polyurethane cover is easily tornor worn by the impact or contact with the clubface. In extreme cases,the solid core would be exposed.

In the following, the solid core used in the present invention will bedescribed. The solid core has no limitation on its structure, and mayhave single-layered structure, or multi-layered structure whichcomprises a center and at least one intermediate layer covering thecenter. The solid core with single-layered structure or the center ofthe multi-layered solid core, for example, without limitation, ispreferably a molded body which is formed by vulcanizing a rubbercomposition. The rubber composition preferably comprises a base rubber,an organic peroxide as a crosslinking agent, and a co-crosslinkingagent. Examples of the base rubber are butadiene rubber (BR),ethylene-propylene-diene terpolymer (EPDM), isoprene rubber (IR),styrene-butadiene rubber (SBR), and acrylonitrile-butadiene rubber(NBR). Among them, butadiene rubber, particularly cis-1,4-polybutadiene,is preferable in view of its superior repulsion property. A preferredembodiment of the present invention employs a high cis-polybutadienerubber having cis-1,4 bond in a proportion of not less than 40%,preferably not less than 70%, more preferably not less than 90%.

Examples of the organic peroxide for use in the present invention aredicumyl peroxide, 1,1-bis(t-butylperoxy)-3,5-trimethylcyclohexane,2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Amongthem, dicumyl peroxide is preferable. The amount of the organic peroxideto be blended in the rubber composition is preferably from 0.2 to 1.5parts by mass, more preferably from 0.3 to 1.0 parts by mass based on100 parts by mass of the base rubber.

The co-crosslinking agent used in the present invention includes, forexample, an α,β-unsaturated carboxylic acid or a metal salt thereof.Typically preferred is the α,β-unsaturated carboxylic acid having 3 to 8carbon atoms such as acrylic acid and methacrylic acid or the metal saltthereof. As the metal forming the metal salt of the α,β-unsaturatedcarboxylic acid, a monovalent or divalent metal such as zinc, magnesium,calcium, aluminum and sodium is preferably used. Among them, zinc ispreferable, because it can impart the higher repulsion property to thegolf ball. The amount of the co-crosslinking agent to be blended in therubber composition is preferably from 20 to 50 parts by mass, morepreferably 25 to 40 parts by mass based on 100 parts by mass of the baserubber.

The rubber composition used in the present invention may further containan organic sulfur compound in addition to the base rubber, theco-crosslinking agent, and the crosslinking agent. Examples of theorganic sulfur compound is a diphenyl disulfide derivative representedby the following formula (2):

wherein “X” represents any one selected from the group consisting of ahydrogen atom, a fluorine atom, a chlorine atom, a bromine atom, aniodine atom, and a cyano group, and each of X may be same as ordifferent from other. Examples of the preferable organic sulfur compoundare diphenyldisulfide, or pentachloro diphenyl disulfide.

As required, the rubber composition may further contain a specificgravity adjusting agent such as zinc oxide and barium sulfate, anantioxidant, a color powder, and the like.

The solid core with single-layered structure or the center is formed bypress-molding the above rubber composition into the spherical body atthe heating conditions. The conditions for the press-molding should bedetermined depending on the rubber composition, but the press-molding ispreferably carried out for 10 to 60 minutes at the temperature of 130 to200° C. Alternatively, the press-molding is preferably carried out in atwo-step heating, for example, for 20 to 40 minutes at the temperatureof 130 to 150° C., and continuously for 5 to 15 minutes at thetemperature of 160 to 180° C.

In the case of the multi-layered solid core, the intermediate layercovering the center has no limitation on its raw material. For example,a rubber composition, or a thermoplastic resin can be used. As a rubbercomposition for the intermediate layer, the rubber composition describedabove for the solid core or the center can be used. In the case of usingthe thermoplastic resin for the intermediate layer, an ionomer resin, apolyester elastomer, a polyurethane elastomer, a polystyrene elastomer,or a polyamide elastomer can be used. The ionomer resin includes“Himilan” available from Mitsui-Dupont Co., “Surlyn” available fromDupont Co., “Iotex” available from Exxon and the like.

An example of the polyester elastomer is “HYTREL” available from DUPONT-TORAY Co., LTD. An Example of the polyurethane elastomer is“ELASTOLLAN” commercially available from BASF POLYURETHANE ELASTOMERSCo. An example of the polystyrene elastomer is “Septon” available fromKuraray Co. An example of the polyamide elastomer is “PEBAX”commercially available from Toray Industries, Inc. The abovethermoplastic resin and elastomeres can be used individually or as amixture of some of them.

Among them, the ionomer resin is particularly preferable because ofimparting superior repulsion property and durability. Examples of theionomer resin are one prepared by neutralizing at least a part ofcarboxyl groups in a copolymer composed of olefin and α,β-unsaturatedcarboxylic acid with a metal ion, and one prepared by neutralizing atleast a part of carboxyl groups in a terpolymer composed of olefin,α,β-unsaturated carboxylic acid and α,β-unsaturated carboxylic acidester with a metal ion. Examples of the α,β-unsaturated carboxylic acidare acrylic acid, and methacrylic acid. Examples of the α,β-unsaturatedcarboxylic acid ester are methyl ester, ethyl ester, propyl ester,n-butyl ester, isobutyl ester and the like of acrylic acid, methacrylicacid. The metal for neutralizing the carboxyl group includes alkalimetal such as sodium, potassium, and lithium; or alkali earth metal suchas magnesium, calcium; or divalent transition metal such as zinc, andcopper. Further, the above ionomer resin can be used as the mixturethereof in order to obtain the desired resilience and hardness.

The intermediate layer may further include a filler such as bariumsulfate, a colorant such as titanium oxide, and an additive such as adispersant, an antioxidant, an UV absorbent, a light stabilizer, and afluorescent brightener, in addition to the base component such as therubber, and the thermoplastic resin.

In the case of using the thermoplastic resin, the method to cover thecenter with the intermediate layer, without limitation, includes aninjection-molding method or a compression-molding method, by which it ispossible to cover the center with the thermoplastic resin. In the caseof using the rubber composition for the intermediate layer, the centeris covered with the rubber composition and then subjected to thevulcanizing-molding.

The intermediate layer has preferably a Shore D hardness of not lessthan 30, more preferably not less than 35, and a Shore D hardness of notmore than 80, more preferably not more than 75. If the intermediatelayer has a Shore D hardness of less than 30, the intermediate layer istoo soft, resulting in lowering the resilience. On the other hand, ifthe intermediate layer has a Shore D hardness of more than 80, theintermediate layer becomes too hard, resulting in lowering the shotfeeling.

Further, the intermediate layer preferably has a thickness of not lessthan 0.5 mm, more preferably not less than 0.7 mm, and a thickness ofnot more than 4.5 mm, more preferably no more than 4 mm. If thethickness is less than 0.5 mm, the resilience becomes low, while if thethickness is larger than 4.5 mm, the shot feeling becomes bad.

The solid core used in the present invention preferably has a diameterof not less than 39.8 mm, more preferably not less than 40.2 mm, and adiameter of not more than 42.4 mm, more preferably not more than 42.2mm. The solid core preferably has the deformation amount of not lessthan 2.20 mm, more preferably not less than 2.30 mm, and a deformationamount of not more than 3.50 mm, more preferably not more than 3.30 mm,when applying a load from 98N (10 kgf) as an initial load to 1275N (130kgf) as a final load.

In the present invention, since the polyurethane cover is so thin, thehardness of the solid core affect the shot feeling to a considerableextent. If the deformation amount is less than 2.20 mm, a larger impactwill be given to the golfer upon shot, due to the hardness. On the otherhand, if the deformation amount is more than 3.50 mm, the solid corebecomes too soft, resulting in the lower resilience and the heavy shotfeeling upon shot.

Generally, the polyurethane cover is formed with a plurality of dimples.In the present invention, since the polyurethane cover is very thin, thepolyurethane cover becomes extremely thin at the bottom of the dimplewhen the surface of the solid core is flat. As a result, the durabilityof the polyurethane cover is lowered at the bottom of the dimple.Therefore, it is preferable to form the dimple at the surface of thesolid core concavely at the same location where the dimple is formed onthe polyurethane cover. This makes the thickness of the polyurethanecover constant, thereby ensuring the durability of the polyurethanecover at the bottom of the dimple.

In the present invention, the polyurethane cover can be produced using aconventional method for manufacturing the golf ball having thepolyurethane cover. For example, the polyurethane cover composition ischarged into the hemispherical mold where the solid core is held, andthen the hemispherical mold is inverted to mate with anotherhemispherical mold, into which the same polyurethane cover compositionhas been charged. Then, the polyurethane cover composition is cured toform the polyurethane cover. When forming the polyurethane cover, thepolyurethane cover is formed with a multiplicity of dimples at thesurface thereof.

As described above, when forming the dimples at the surface of the solidcore, it is preferable to form the dimples of the polyurethane cover onthe same locations where the dimples are formed at the surface of thesolid core. If the concavity and convexity (dimples) are formed at theboth surfaces of the solid core and the mold, since the solid core isfixed during the formation of the polyurethane cover, it becomes easy tomake the thickness of the polyurethane cover constant, compared with thecase that the solid core has no concavity and convexity on the surfacethereof. Further, the golf ball of the present invention is usuallyprovided with a paint finish, a marking stamp, or the like when launchedinto the market in order to enhance the attractiveness and commercialvalue.

The golf ball configured as described above, preferably has thedeformation amount of at least 2.20 mm, more preferably at least 2.30mm, and the deformation amount of not more than 3.50 mm, more preferablynot more than 3.30 mm when applying a load from 98N(10 kgf) as aninitial load to 1275 N (130 kgf) as a final load to the golf ball. Ifthe deformation is less than 2.20 mm, the golf ball becomes excessivelyhard, resulting in the hard shot feeling. While if the deformation islarger than 3.50 mm, the golf ball becomes excessively soft, resultingin the low resilience. Further, the shot feeling becomes heavy.

EXAMPLE

The following examples illustrate the present invention, however theseexamples are intended to illustrate the invention and are not to beconstrued to limit the scope of the present invention. Many variationsand modifications of such examples will exist without departing from thescope of the inventions. Such variations and modifications are intendedto be within the scope of the invention.

[Measurement and Evaluation]

1. Pot Life (Seconds)

The isocyanate group terminated polyurethane prepolymer was heated to80° C., and then mixed with each curing agent which is heated at thepredetermined temperature (Curing agent adding temp.) to prepare thepolyurethane cover composition. The time period between the time theisocyanate group terminated polyurethane prepolymer and4,4′-diaminodiphenylmethane derivative were mixed and the time when theviscosity of the polyurethane cover composition reached 20,000 mPa·s(cPs) was measured by using a Brookfield viscosity meter (spindle No. 4,12 rpm).2. Compression Deformation (mm)

The deformation amount (mm) of the golf balls or the solid cores wasmeasured when applying a load from 98N(10 kgf) as an initial load to1275 N (130 kgf) as a final load to the golf balls or the solid cores.

3. Shore D Hardness of the Polyurethane Cover

Each of the polyurethane cover compositions was formed into sheets eachhaving a thickness of approximately 2 mm by hot-press-molding. Theresulting sheets were stored for 2 weeks at 23° C. At least three of theresulting sheets were stacked one upon another, so as to avoid beingaffected by the measuring substrate on which the sheets were placed, andthe stack were subjected to the measurement using a spring-type Shore Dhardness tester stipulated in ASTM-D2240.

4. Repulsion Index

Each golf ball was struck with a 198.4 g aluminum cylinder at the speedof 45 m/sec., the speed of the cylinder and the golf ball wererespectively measured before and after being struck, and the repulsioncoefficient of the each golf ball was calculated based on the speeds andweight of the cylinder and the golf balls. The measurement was carriedout 5 times, and the average of 5 times was regarded as the repulsioncoefficient of the golf ball. The repulsion coefficient of the golf ballwas represented as an index relative to the repulsion coefficient of thegolf ball No. 12 which is represented as 100. The larger repulsion indexmeans the higher resilience.

5. Spin Rate (rpm)

Each golf ball was hit with a titanium driver (W#1) attached to a swingrobot manufactured by Golf laboratory Co. at the head speed of 45m/sec., and the spin rate (rpm) was determined by taking a photograph ofthe spinning golf ball two times.

Each golf ball was hit with a sand wedge club attached to a swing robotmanufactured by Golf laboratory Co. at the head speed of 21 m/sec., andthe spin rate (rpm) was determined by taking a photograph of thespinning golf ball two times.

6. Flight Distance (m)

Each golf ball was hit with a titanium driver (W#1) attached to a swingrobot, at the head speed of 45 m/sec. The flight distance from thehitting point to the point where the golf ball fell to the ground wasmeasured.

7. Abrasion Resistance

Each golf ball was hit with a commercially available pitching wedgeattached to a swing robot at the head speed of 36 m/sec. The conditionsof the golf ball thus hit was visually observed and rated on four levelsaccording to the following criteria.

Rating Criteria:

“E(Excellent)”: The surface of the golf ball had few scratches.

“G(Good)”: The surface of the golf ball had a few scratches, which wereat an unnoticeable degree;

“F(Fair)”: The surface of the golf ball had clearly noticeable scratchesand slight nap; and

“P(Poor)”: The surface of the golf ball was considerably shaved andconspicuously napped.

8. Coloring-Resistance Against Weather

Each golf ball was subjected to the weather-resistance test by using“Sunshine Super Long life Weather Meter” manufactured by Suga TestInstruments Co., LTD., according to JIS D 0205 under the conditions:chamber temperature=63° C., humidity=50%, irradiation=60 hours, and 12minutes' rainfall per 60 minutes. The “L”, “a”, and “b” values of thecolor tone of the golf ball before and after the weathering test weremeasured by using color meter (“CR-221 manufactured by Minolta Co.). Thecolor difference was calculated according to the following equation.ΔE*ab=[(ΔL*)²+(Δa*)²+(Δb*)²]^(1/2)The lager value of Δ E*ab indicates the larger difference of the colortone.Manufacture of the Golf Ball1. Production of the Solid Core.

The rubber composition shown in Table 2 was vulcanized and formed at160° C. for 20 minutes to obtain the solid core having a diameter of41.2 mm and a weight of 40.9 g. The compressive deformation amount ofthe solid core was 2.83 mm.

TABLE 2 Raw material Part by mass Butadiene Rubber 100 Zinc acrylate 36Zinc oxide 10.4 Dicumyl peroxide 0.7 Diphenyl disulfide 0.5 Note onTable 2: Butadiene rubber: BR-18 (cis content: 96%) available from JSRCo. Zinc acrylate: “ZNDA-90S” produced by NIHON JYORYU KOGYO Co., LTD.Zinc oxide: “Ginrei R” produced by Toho-Zinc Co. Diphenyl disulfide:Diphenyl disulfide produced by Sumitomo Seika Chemicals Co., LTD.Dicumyl peroxide: “Percumyl D” produced by NOF Corporation.2. Production of the Polyurethane CoverAs shown in tables 3 and 4, the isocyanate group terminated polyurethaneprepolymer heated at 80° C. were mixed with each curing agent heated atthe predetermined temperature (Curing agent adding temperature) toprepare a polyurethane cover composition.

Each solid core obtained above was held in the hemispherical mold, andthen the polyurethane cover composition was charged into thehemispherical mold. The hemispherical mold was inverted to mate withanother hemispherical mold (having a convexity for forming dimples),into which the same polyurethane cover composition had been charged.Then, the polyurethane cover composition was cured to form thepolyurethane cover at 80° C. for 30 minutes. After curing thepolyurethane cover composition, the obtained golf ball was dischargedfrom the mold, deburred, painted with a white paint at the surface, andthen painted with a clear coating to obtain two-piece golf balls havinga diameter of 42.8 mm and a weight of 45.2 to 45.7 g. The obtained golfballs were evaluated in terms of the compressive deformation amount,repulsion index, abrasion-resistance, flight distance, spin rate, andshot feeling. The results were also shown in Tables 3 and 4. In tables,the amount of the formulation was shown by mass parts. Furthermore, thedetails of the isocyanate group terminated polyurethane prepolymer andcuring agents, which were used, are summarized below.

Adiprene LW520: dicyclohexylmethanediisocyanate (Hydrogenated MDI)-PTMGisocyanate group terminated polyurethane prepolymer having more than0.5% of free Hydrogenated MDI and NCO content of 4.8%, available fromUniroyal Co.

Adiprene LW570: dicyclohexylmethanediisocyanate (Hydrogenated MDI)-PTMGisocyanate group terminated prepolymer having more than 0.5% of freeHydrogenated MDI and NCO content of 7.5%, available from Uniroyal Co.

Urethane prepolymer A: IPDI-PTMG isocyanate group terminatedpolyurethane prepolymer having more than 0.5% of free IPDI and NCOcontent of 8.0%.

Urethane prepolymer B: IPDI-PTMG isocyanate group terminatedpolyurethane prepolymer having more than 0.5% of free IPDI and NCOcontent of 12.0%.

Adiprene LF950A: Toluenediisocyanate(TDI)-PTMG isocyanate groupterminated polyurethane prepolymer having more than 0.1% of free TDI andNCO content of 6.1%, available from Uniroyal Co.

Vibrathane B635:4,4′-diphenylmethanediisocyanate (MDI)-PTMG isocyanategroup terminated polyurethane prepolymer having more than 0.5% of freeMDI and NCO content of 7.8%, available from Uniroyal Co.

Curehard MED:3,3′-diethyl-5,5′-dimethyl-4,4′-diaminodiphenylmethanehaving amine value of 398 mgKOH/g, available from IHARA CHEMICALINDUSTRY Co., LTD. (Curing agent adding temperature:100° C.)3,3′-dimethyl-4,4′-diaminodiphenylmethane available from Tokyo kaseiKogyo Co., LTD (Curing agent adding temperature: 150° C.).VersalinkP-250:Polytetramethyleneoxide-di-paraminobenzoa te having anamine value of 250 mgKOH/g produced by Air products Co.(Curing agentadding temperature: 60° C.)Ethacure 100: 3,5-diethyltoluenediamine having an amine value of 630mgKOH/g, available from Albemalre Co. (Curing agent adding temperature:30° C.)Ethacure 300: 3,5-dimethylthiotoluenediamine having an amine value of530 mgKOH/g, available from Albemalre Co; (Curing agent addingtemperature: 30° C.)LONZACURE M-CDEA:4,4′-methylenebis(3-chloro-2,6′-diethylaniline) havingan amine value of 297 mgKOH/g, available from Lonza Co. (Curing agentadding temperature: 100° C.)

TABLE 3 Golf ball No. 1 2 3 4 5 6 Polyurethane — — — — — — covercomposition NCO — — — — — — terminated Pu prepolymer Adiprene 100 75 50— — 75 LW520 Adiprene — 25 50 — — 25 LW570 Urethane — — — 100 — —prepolymer A Urethane — — — — 100 — prepolymer B Curing agent — — — — —— Cure hard 15.3 17.5 19.6 25.5 31.4 — MED (100° C.) 3,3′-dimethyl- — —— — — 14.0 4,4′-diaminodi- phenylmethane (150° C.) Titanium oxide 2 2 22 2 2 Pot life (sec.) 360 330 300 100 80 210 Shore D 41 51 58 52 59 50harness Properties of Golf ball Deformation 2.77 2.72 2.66 2.66 2.632.74 amount (mm) Repulsive 102 103 105 104 106 103 Index Flight distance231 234 236 234 237 233 (m)/W#1 Spin rate 3150 3070 2950 3030 2930 3090(rpm)/W#1 Spin rate 6890 6780 6580 6700 6550 6810 (rpm)/SW Abrasion G EE G G E resistance Coloring 1.8 2.4 2.8 3.4 3.6 2.3 resistance (ΔE)

Golf ball Nos. 1 to 6 used a polyurethane cover composition whichincludes the isocyanate group terminated polyurethane prepolymercontaining at least one diisocyanate selected from the group consistingof 4,4′-dicyclohexylmethanediisocyanate, isophoronediisocyanate andcyclohexanediisocyanate as a polyisocyanate component and4,4′-diaminodiphenylmethane derivative used in the present invention.The polyurethane cover compositions had pot lives of 80 to 360 seconds.Since the polyurethane cover compositions had a suitable pot lives, theworkability for covering the solid core with the polyurethane cover wasgood.

The obtained golf ball was each excellent in resilience, flight distance(231 to 234 m), and abrasion-resistance, coloring-resistance.Especially, the coloring-resistance(Δ E) became very small, because itis considered that the alicyclic diisocyanate was used as thepolyisocyanate component of the isocyanate group terminated polyurethaneprepolymer. Further, the use of 4,4′-diaminodiphenylmethane derivativeas the curing agent enhanced the cohesive energy, which improved themechanical properties of the polyurethane cover. Thus, theabrasion-resistance of the obtained polyurethane cover was improved. Inaddition, the shot feeling of each golf ball was good.

TABLE 4 Golf ball No. 7 8 9 10 11 12 13 14 15 Polyurethane covercomposition — — — — — — — — — NCO terminated Pu prepolymer — — — — — — —— — Adiprene LW520 100 100 100 100 — — — — — Vibrathane B635 — — — — 100100 — — — Adiprene LF950A — — — — — — 100 100 100 Curing Agent — — — — —— — — — Cure Hard MED (100° C.) — — — — 24.7 — 19.5 — — Versa link P250(60° C.) 24.4 — — — — 39.3 — — Ethacure 100 (30° C.) — 9.7 — — — — — 6.7— Ethacure 300 (30° C.) — — 11.5 — — — — 6.7 — Lonza cure M-CDEA (100°C.) — — — 20.5 — — — — 26.1 Titanium oxide 2 2 2 2 2 2 2 2 2 Pot life(sec.) 3300 320 1680 2100 — 200 25 210 300 Shore D hardness — 42 — — —46 — 48 52 Properties of Golf ball Deformation amount (mm) — 2.75 — — —2.74 — 2.70 2.68 Repulsion Index — 104 — — — 100 — 101 101 Fightdistance (m)/W#1 — 234 — — — 225 — 229 230 Spin rate (rpm)/W#1 — 2950 —— — 3320 — 3120 3060 Spin rate (rpm)/SW — 6760 — — — 6740 — 6600 6570Abrasion resistance — F — — — P — F G Coloring resistance (ΔE) — 4.2 — —— 5.8 — 8.3 7.0

In golf ball Nos. 7 to 10, the curing agent other than4,4′-diaminodiphenylmethane derivative used in the present invention wasemployed for the polyurethane cover composition. In golf ball No. 7,9and 10, the reactivity of the curing agent to the isocyanate groupterminated polyurethane prepolymer used in the present invention was solow, thus the pot life of the polyurethane cover composition was long.As a result, it was impossible to obtain the golf ball by curing thepolyurethane cover composition. Golf ball No. 11 and 13 are the caseswhere the aromatic polyisocyanate was used as the polyisocyanatecomponent of the isocyanate group terminated polyurethane prepolymer incombination with the 4,4′-diaminodiphenylmethane derivative used in thepresent invention. In this combination, the reactivity of the aminogroup of the curing agent is too high to the isocyanate group, thus thepot life became extremely short. In golf ball No. 11, the pot life couldnot be measured. The pot life of the golf ball No. 13 was 25 seconds. Inboth cases, the polyurethane cover could not be formed, and finally thegolf ball could not be obtained, because the pot life was too short.

In terms of the golf balls No. 8, No. 12 and No. 14, the pot lives ofthe polyurethane cover compositions ranged from 200 to 320 seconds,which fell within the suitable range. However, since the curing agentother than 4,4′-diaminodiphenylmethane derivative was used, theabrasion-resistance was lowered, due to the low cohesive energy.Further, in the golf balls No. 12, No. 14 and No. 15, the aromaticpolyisocyanate was used as the polyisocyanate component of theisocyanate group terminated polyurethane prepolymer, thus thecoloring-resistance was remarkably lowered.

The golf ball of the present invention provides an excellentcoloring-resistance, since the specified polyisocyanate is used as thepolyisocyanate component of the isocyanate group terminated polyurethaneprepolymer. Further, the productivity of the golf ball was improved bycontrolling the pot life of the polyurethane cover composition, with theuse of the specified 4,4′-diamidnodiphenylmethane derivative as thecuring agent for the isocyanate group terminated polyurethaneprepolymer. Furthermore, since 4,4′-diaminodiphenylmethane derivativeprovides the resultant polyurethane with the high cohesive energy, theabrasion-resistance of the polyurethane cover is also improved.

1. A golf ball having a polyurethane cover, wherein the polyurethanecover is formed by curing a polyurethane cover composition whichcomprises: an isocyanate group terminated polyurethane prepolymercomprising 4,4′-dicyclohexylmethanediisocyanate as a diisocyanatecomponent and a polyoxytetramethyleneglycol as a polyol component,wherein the polyurethane prepolymer contains 0.5% or more of free4,4′-dicyclohexylmethanediisocyanate, and3,3-diethyl-55′-dimethyl-4,4′-diaminodiphenylmethane.
 2. The golf ballaccording to claim 1, wherein the polyurethane cover composition has apot life of 30 to 900 seconds.
 3. The golf ball according to claim 1,wherein the polyurethane cover has a Shore D hardness of from 30 to 60.4. The golf ball according to claim 1, wherein the polyurethane coverhas a thickness of 0.2 mm to 1.5 mm.
 5. A two-piece golf ball having asolid core and a polyurethane cover covering the solid core, wherein thepolyurethane cover is formed by curing a polyurethane cover compositionwhich comprises: an isocyanate group terminated polyurethane prepolymercomprising 4,4′-dicyclohexylmethanediisocyanate as a diisocyanatecomponent, and a polyoxytetramethyleneglycol as a polyol component,wherein the polyurethane prepolymer contains 0.5% or more of free4,4′-dicyclohexylmethanediisocyanate, and3,3-diethyl-5,5′-dimethyl-4,4′-diaminodiphenylmethane.
 6. The golf ballaccording to claim 5, wherein the polyurethane cover composition has apot life of 30 to 900 seconds.
 7. The golf ball according to claim 5,wherein the polyurethane cover has a Shore D hardness of from 30 to 60,and a thickness of 0.2 mm to 1.5 mm.
 8. The golf ball according to claim5, wherein the solid core is formed by vulcanizing a rubber compositionwhich comprises: a base rubber; an organic peroxide; a co-crosslinkingagent; and a diphenyl disulfide derivative represented by the followingformula (2):

wherein X represents any one selected from the group consisting of ahydrogen atom, a fluorine atom, a chlorine atom, a bromine atom, aniodine atom, and a cyano group, and each X may be the same as ordifferent from each other X.
 9. A multi-piece golf ball having a solidcore including a center and at least one intermediate layer covering thecenter; and a polyurethane cover covering the solid core, wherein thepolyurethane cover is formed by curing a polyurethane cover compositionwhich comprises: an isocyanate group terminated polyurethane prepolymercomprising 4,4′-dicyclohexylmethanediisocyanate as a diisocyanatecomponent, and a polyoxytetramethyleneglycol as a polyol component,wherein the polyurethane prepolymer contains 0.5% or more of free4,4′-dicyclohexylmethanediisocyanate, and3,3-diethyl-5,5′-dimethyl-4,4′-diaminodiphenylmethane.
 10. The golf ballaccording to claim 9, wherein the polyurethane cover composition has apot life of 30 to 900 seconds.
 11. The golf ball according to claim 9,wherein the polyurethane cover has a Shore D hardness of from 30 to 60and a thickness of 0.2 mm to 1.5 mm.
 12. The golf ball according toclaim 9, wherein the center is formed by vulcanizing a rubbercomposition comprising: a base rubber; an organic peroxide; aco-crosslinking agent; and a diphenyl disulfide derivative representedby the following formula (2):

wherein X represents any one selected from the group consisting of ahydrogen atom, a fluorine atom, a chlorine atom, a bromine atom, aniodine atom, and a cyano group, and each X may be the same as ordifferent from each other X.
 13. The golf ball according to claim 9,wherein the intermediate layer comprises at least one selected from thegroup consisting of an ionomer resin, a polyester elastomer, apolyurethane elastomer, a polystyrene elastomer, and a polyamideelastomer, and wherein the intermediate layer has a Shore D hardness offrom 30 to 75.